Sarah Brown's memoirs to reveal influence and role

One would have thought a former career in public relations and a familiarity with the machinations of the Press might have helped her to easily avoid a blooper.

But Sarah Brown, despite her reputation as an experienced and professional PR woman, broke her own publisher's embargo yesterday by announcing she had agreed a deal to write her memoirs.

The deal was to be announced this morning, and press releases were sent out yesterday with a not for publication "before 0001hrs Monday 14th June" heading, but any attempts at secrecy were blown when Mrs Brown, the former Prime Minister's wife, declared the agreement to her 1.1 million followers on Twitter.

In the 24-word Tweet just before 11am, in which she mis-spelt 'publisher', she said she would begin writing the book today: "has just agreed with my pubilsher Ebury to write a book – working title Behind the Black Door – so chained to laptop from Monday morning".

Only an hour earlier the publisher had issued a press release detailing the memoirs deal in a press release marked "strictly embargoed".

Mrs Brown's book is intended to reveal what life was like as Britain's first lady during the two years and 10 months her husband was prime minister. It promises to be more of a Wag's-eye-view of first lady life than an analysis of where it all went wrong for Gordon.

Despite being married to one of the political heavyweights of the modern era, Mrs Brown's memoirs are said to concentrate more on hair-dos, shopping and school visits than on policy or the machinations of government.

She will write, a statement by the publisher said, about "what it's like to shop with Special Branch; overcome stomach-churning nerves at your first major charity event, or cope with a bad hair day when Carla Bruni's in town."

It will look at Mrs Brown's transformation from a career woman to being a mother and wife who, while remaining largely in the background, was a strong influence on her husband. It will also chart the challenges of taking on the role of a campaigner for charities, including PiggyBankKids, where she is president, Wellbeing of Women and Women's Aid.

The former prime minister has been seeking a publisher for his own memoirs, and is said to have had trouble generating interest. His wife was facing similar difficulties until she secured her deal with Ebury Press.

Details of the contract remained confidential yesterday, but it is thought it would be worth a six-figure sum to Mrs Brown, who gave up her PR career to be a mother and to support her husband. She describes herself on Twitter as a "bio mum" and "starting a new life".

Fiona MacIntyre, managing director of Ebury Publishing, said: "Sarah has achieved so much in her own right, with opportunities that allowed her to meet a wide range of women.

"People like and admire her, and one of the defining images was Sarah leaving Downing Street with Gordon and their two small sons with grace and dignity."

The book, which is expected to be published on Mother's Day 2011, is likely to be beaten into the bookshops by memoirs from other senior Labour Party figures.

Among them is Lord Mandelson whose recollections The Third Man: Life at The Heart of New Labour are believed to have been signed up in a deal worth £1m and are expected to be issued this summer.

Alastair Campbell, the former Blairite spin doctor, will have an updated version of his memoirs published, and in the autumn Tony Blair's The Journey, for which he was paid £4.6m, will be available.